If you have worded the question verbatim from the problem, then you only need to worry about the 4.2% paid by the employee out of his/her gross.
(If you altered the wording a bit, then the 6.2% from the employer may factor in somehow. But the way the problem is worded above, it does not.)
If I make $28,000 and paid 4.2% out of my wages myself, then that would be
28,000 x .042 = $1176 toward SS that I paid myself from my gross wages.